Systems and methods for optimizing magnetic torque and pulse shaping for reducing write error rate in magnetoelectric random access memory

ABSTRACT

Systems and methods for reducing write error rate in MeRAM applications in accordance with various embodiments of the invention are illustrated. One embodiment includes a method for a writing mechanism for a magnetoelectric random access memory cell, the method including applying a voltage of a given polarity for a given period of time across a magnetoelectric junction bit of the magnetoelectric random access memory cell, wherein application of the voltage of the given polarity across the magnetoelectric junction bit reduces the perpendicular magnetic anisotropy and magnetic coercivity of the ferromagnetic free layer through a voltage controlled magnetic anisotropy effect, and lowering the applied voltage of the given polarity before the end of the given period of time, wherein the given period of time is approximately half of a precessional period of the ferromagnetic free layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a continuation in part of U.S. patentapplication Ser. No. 16/020,933 entitled “Systems and Methods forReducing Write Error Rate in Magnetoelectric Random Access MemoryThrough Pulse Sharpening and Reverse Pulse Schemes,” filed Jun. 27,2018, which claims priority to U.S. Provisional Patent Application No.62/525,661 entitled “A Reverse Pulse (RVP) Scheme in MagnetoelectricRandom Access Memory (MeRAM) for Reducing Write Error Rate,” filed Jun.27, 2017. The disclosures of U.S. patent application Ser. No. 16/020,933and U.S. Provisional Patent Application No. 62/525,661 are herebyincorporated by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention generally relates to magnetoelectric random accessmemory and, more specifically, to writing schemes for magnetoelectricrandom access memory.

BACKGROUND

Devices that rely on electricity and magnetism underlie much of modernelectronics. Researchers have recently begun to develop and implementdevices that take advantage of both electricity and magnetism inspin-electronic (or so-called “spintronic”) devices. These devicesutilize quantum-mechanical magnetoresistance effects, such as giantmagnetoresistance (“GMR”) and tunnel magnetoresistance (“TMR”). GMR andTMR principles regard how the resistance of a thin film structure thatincludes alternating layers of ferromagnetic and non-magnetic layersdepends upon whether the magnetizations of ferromagnetic layers are in aparallel or antiparallel alignment. For example, magnetoresistiverandom-access memory (“MRAM”) is a technology that is being developedthat typically utilizes TMR phenomena in providing for alternativerandom-access memory (“RAM”) devices. In a typical MRAM bit, data isstored in a magnetic structure that includes two ferromagnetic layersseparated by an insulating layer—this structure is conventionallyreferred to as a magnetic tunnel junction (“MTJ”). The magnetization ofone of the ferromagnetic layers (the fixed layer) is permanently set toa particular direction, while the other ferromagnetic layer (the freelayer) can have its magnetization direction free to change. Generally,the MRAM bit can be written by manipulating the magnetization of thefree layer such that it is either parallel or antiparallel with themagnetization of the fixed layer; and the bit can be read by measuringits resistance (since the resistance of the bit will depend on whetherthe magnetizations are in a parallel or antiparallel alignment).

MRAM technologies initially exhibited a number of technologicalchallenges. The first generation of MRAM utilized the Oersted fieldgenerated from current in adjacent metal lines to write themagnetization of the free layer, which required a large amount ofcurrent to manipulate the magnetization direction of the bit's freelayer when the bit size shrinks down to below 100 nm. Thermal assistedMRAM (“TA-MRAM”) utilizes heating of the magnetic layers in the MRAMbits above the magnetic ordering temperature to reduce the write field.This technology also requires high power consumption and long wirecycles. Spin transfer torque MRAM (“STT-MRAM”) utilizes thespin-polarized current exerting torque on the magnetization direction inorder to reversibly switch the magnetization direction of the freelayer. The challenge for STTMRAM remains that the switching currentdensity needs to be further reduced.

SUMMARY OF THE INVENTION

Systems and methods for reducing write error rate in MeRAM applicationsin accordance with various embodiments of the invention are illustrated.One embodiment includes a method for a writing mechanism for amagnetoelectric random access memory cell, the method including applyinga voltage of a given polarity for a given period of time across amagnetoelectric junction bit of the magnetoelectric random access memorycell, wherein the magnetoelectric junction bit includes a ferromagneticfree layer, a ferromagnetic fixed layer, and a dielectric layerinterposed between the ferromagnetic free layer and the ferromagneticfixed layer, application of the voltage of the given polarity across themagnetoelectric junction bit reduces the perpendicular magneticanisotropy and magnetic coercivity of the ferromagnetic free layerthrough a voltage controlled magnetic anisotropy effect, and themagnetization of the ferromagnetic free layer changes direction inresponse to the application of the voltage of the given polarity for thegiven period of time, and lowering the applied voltage of the givenpolarity before the end of the given period of time, wherein the givenperiod of time is approximately half of a precessional period of theferromagnetic free layer.

In another embodiment, the magnetoelectric random access memory cellincludes a first terminal coupled to a bit line, a second terminalcoupled to a source line, and a third terminal coupled to a word line,and the magnetoelectric junction bit is coupled to the drain of an MOStransistor.

In a further embodiment, the method further includes applying a voltageof a polarity opposite the given polarity across the magnetoelectricjunction bit at the end of the application of the voltage of the givenpolarity.

In still another embodiment, the voltage of the given polarity isapplied using a pulse generator.

In a still further embodiment, the pulse generator is selected from thegroup of a bit line driver, a source line driver, and a word linedriver.

In yet another embodiment, the rising edge of the application of thevoltage of the given polarity decreases the perpendicular magneticanisotropy and causes a precessional motion of magnetization between twostates of the ferromagnetic free layer, the magnetization direction ofthe ferromagnetic free layer is different between the two states, andthe falling edge of the application of the voltage of the given polarityrestores the decrease in the perpendicular magnetic anisotropy and stopsthe precessional motion of magnetization.

In a yet further embodiment, the method further includes applying avoltage of a polarity opposite the given polarity across themagnetoelectric junction bit at the end of the application of thevoltage of the given polarity, wherein the voltage of the polarityopposite the given polarity is applied across the magnetoelectricjunction bit subsequent or near simultaneously with the falling edge ofthe application of the voltage of the given polarity to increase theperpendicular magnetic anisotropy of the ferromagnetic free layer.

In another additional embodiment, the voltage of the polarity oppositethe given polarity is applied using capacitive coupling from the wordline to the magnetoelectric junction bit through thegate-to-source-capacitance.

In a further additional embodiment, the voltage of the polarity oppositethe given polarity is applied through generating a negative voltage withrespect to ground on the bit line while keeping a voltage of the sourceline at ground level.

In another embodiment again, the voltage of the polarity opposite thegiven polarity is applied by generating a positive voltage pulse on thesource line after a write voltage pulse on the bit line.

A further embodiment again includes a magnetoelectric random accessmemory cell including a magnetoelectric junction bit including aferromagnetic free layer, a ferromagnetic fixed layer, and a dielectriclayer interposed between the ferromagnetic free layer and theferromagnetic fixed layer, wherein the magnetoelectric junction bit isconfigured such that when a voltage of a given polarity is appliedacross the magnetoelectric junction bit for half a precessional periodof the ferromagnetic free layer, the perpendicular magnetic anisotropyand magnetic coercivity of the ferromagnetic free layer are reducedthrough a voltage controlled magnetic anisotropy effect and themagnetization of the ferromagnetic free layer changes direction, andwherein the magnetoelectric junction bit is configured such that whenthe applied voltage of the given polarity is reduced before the end ofthe half precessional period of the ferromagnetic free layer, a magnetictorque of the ferromagnetic free layer is reduced before a maximummagnetic trajectory of the ferromagnetic free layer is reached.

In still yet another embodiment, the magnetoelectric random accessmemory cell further includes a first terminal coupled to a bit line, asecond terminal coupled to a source line, and a third terminal coupledto a word line.

In a still yet further embodiment, the magnetoelectric junction bit iscoupled to the drain of an MOS transistor.

In still another additional embodiment, the voltage of the givenpolarity is applied using a pulse generator.

In a still further additional embodiment, the pulse generator isselected from the group of a bit line driver, a source line driver, anda word line driver.

In still another embodiment again, the rising edge of the application ofthe voltage of the given polarity decreases the perpendicular magneticanisotropy and causes a precessional motion of magnetization between twostates of the ferromagnetic free layer, wherein the magnetizationdirection of the ferromagnetic free layer is different between the twostates, and the falling edge of the application of the voltage of thegiven polarity restores the decrease in the perpendicular magneticanisotropy and stops the precessional motion of magnetization.

In a still further embodiment again, a voltage of the polarity oppositethe given polarity is applied across the magnetoelectric junction bitsubsequent or near simultaneously with the falling edge of theapplication of the voltage of the given polarity to increase theperpendicular magnetic anisotropy of the ferromagnetic free layer.

In yet another additional embodiment, the voltage of the polarityopposite the given polarity is applied using capacitive coupling fromthe word line to the magnetoelectric junction bit through thegate-to-source-capacitance.

In a yet further additional embodiment, the voltage of the polarityopposite the given polarity is applied through generating a negativevoltage with respect to ground on the bit line while keeping a voltageof the source line at ground level.

In yet another embodiment again, the voltage of the polarity oppositethe given polarity is applied by generating a positive voltage pulse onthe source line after a write voltage pulse on the bit line.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention.

FIG. 1 conceptually illustrates the voltage dependence of the PMA viathe VCMA effect in accordance with an embodiment of the invention.

FIGS. 2A and 2B conceptually illustrate a write process and a graphdepicting the associated energy barrier of an MEJ in accordance with anembodiment of the invention.

FIGS. 3A and 3B conceptually illustrate the oscillations of theresistance of an MTJ after the removal of an applied voltage inaccordance with an embodiment of the invention.

FIG. 4A conceptually illustrates an MEJ whereby the FM fixed layer andthe FM free layer are separated by, and directly adjoined to, adielectric layer in accordance with an embodiment of the invention.

FIG. 4B conceptually illustrates an MEJ with constituent cap/seed layersin accordance with an embodiment of the invention.

FIG. 5 conceptually illustrates an MEJ whereby the orientation of themagnetization directions is perpendicular to the plane of theconstituent layers in accordance with an embodiment of the invention.

FIG. 6 conceptually illustrates an MEJ that includes multiple layersthat work in aggregate to facilitate the functionality of the MEJ inaccordance with an embodiment of the invention.

FIGS. 7A and 7B conceptually illustrate MEJs that include a semi-fixedlayer in accordance with various embodiments of the invention.

FIGS. 8A and 8B conceptually illustrate how the application of apotential difference can reduce the coercivity of the free layer inaccordance with various embodiments of the invention.

FIG. 9 conceptually illustrates using a metal line disposed adjacent toan FM free layer to generate spin-orbit torques that can impose amagnetization direction change on the FM free layer in accordance withan embodiment of the invention.

FIG. 10 conceptually illustrates the implementation of two MEJs that arehoused within encapsulating layers and separated by field insulation inaccordance with an embodiment of the invention.

FIG. 11 conceptually illustrates a MeRAM bit-cell in accordance with anembodiment of the invention.

FIG. 12 conceptually illustrates a circuit diagram of MeRAM cells inaccordance with an embodiment of the invention.

FIGS. 13A-13C conceptually illustrate three examples of applying areverse voltage on the MEJ after the write pulse in accordance withvarious embodiments of the invention.

DETAILED DESCRIPTION

Turning now to the drawings, reverse pulse (“RVP”) schemes for reducingwrite error rate (“WER”) in magnetoelectric random access memory(“MeRAM”) applications are illustrated. MeRAM cells in accordance withvarious embodiments of the invention can be implemented withvoltage-controlled magnetic anisotropy-based MTJs (“VMTJs”). A VMTJ canbe referred to as an MTJ that utilizes an electric field to facilitatethe switching of the magnetization direction of the free layer (i.e.,“write” to it) as opposed to (or in some cases, in addition to) using acurrent. Generally, the coercivity of the free layer of a VMTJ can bereduced using voltage-controlled magnetic anisotropy (“VCMA”) phenomena,thereby making the free layer more easily switched to the oppositedirection (i.e., “writeable”). Examples of such applications aredescribed in International Patent Application Number PCT/US2012/038693,entitled “Voltage-Controlled Magnetic Anisotropy (VCMA) Switch andMagneto-electric Memory (MERAM),” by Khalili Amiri et al., thedisclosure of which is herein incorporated by reference in its entirety.

In many embodiments, a MeRAM can be implemented with a magnetoelectricjunction (“MEJ”) bit that operates as a storage element. For thepurposes of describing the invention, MEJ can be used to refer todevices that use VCMA principles to help realize two distinctinformation states. Examples of such devices include but are not limitedto VMTJs and VCMA switches, such as those discussed in InternationalPatent Application Number PCT/US2012/038693. In some embodiments, an MEJincludes at least two ferromagnetic layers divided by a tunnelingbarrier, which can be implemented using a variety of materials such asbut not limited to MgO. The magnetic moment of one layer can switchfreely by using an electric field, electric current, spin torquegenerated by spin-orbit interaction, magnetic field, or a combination ofthese. Typically, the magnetic moment of the other layer is fixed. Theconventional write operation for an MEJ can be implemented by giving asingle voltage pulse from the top electrode (“TE”) to the bottomelectrode (“BE”) of the device. When a pulse is applied to the TE, theVCMA effect can reduce the perpendicular magnetic anisotropy (“PMA”) ofthe free layer. The VCMA effect can be explained in terms of theelectric-field-induced change of occupancy of atomic orbitals at theinterface, which, in conjunction with spin-orbit interaction, results ina change of anisotropy.

FIG. 1 conceptually illustrates the voltage dependence of the PMA viathe VCMA effect in accordance with an embodiment of the invention. Asshown, under zero bias condition (V=0), the PMA of the MEJ is at aresting value H_(C), which can hold the state of the MEJ. By applyingV_(WRITE), the PMA can be reduced to allow precessional switchingbetween the two states. Conversely, a bias of the opposite polarity canincrease the PMA. The slope m of the PMA with respect to voltage isproportional to the VCMA coefficient. With the correct selection ofvoltage polarity, the PMA can be reduced, which lowers the energybarrier of the free layer. The lowered energy barrier can cause aprecessional motion of magnetization that can be observed via aresistance oscillation in the device. As the applied pulse is removed atthe end of the write operation, the PMA returns to its original value atzero bias, retaining the written state.

FIGS. 2A and 2B conceptually illustrate a write process and a graphdepicting the associated energy barrier of an MEJ in accordance with anembodiment of the invention. FIG. 2A shows three time periods during awrite process of the MEJ, while FIG. 2B illustrates the change in theenergy barrier. In the first time period leading up to t₀, there is noapplied voltage (V_(MTJ)=0 V). The device is in its resting state andout-of-plane {right arrow over (H)}_(PMA) is dominant. As shown, in theinitial magnetization state, the magnetic field {right arrow over(H)}_(eff) and moment {right arrow over (m)} is in a particulardirection. In this state, the associated energy barrier is at E_(b0), asshown in FIG. 2B. At time period t₁ through t₂, a voltage VP is appliedacross the device, which reduces the PMA. As a result, the energybarrier is reduced to E_(b), and In-plane {right arrow over (H)}ext isdominant. This allows for precessional switching, or oscillation,between the two states of the MEJ. Once the MEJ state has successfullyswitched, the applied voltage can be removed (V_(MTJ)=0 V) to restorethe energy barrier to E_(b0), which stops the precessional motion. Thisis shown at time t3. In the final magnetization state, magnetic field{right arrow over (H)}_(eff) and moment {right arrow over (m)} are in adifferent direction compared to the initial magnetization state.

Since the VCMA driven precessional motion can cause the deviceresistance to oscillate between the two possible states (high and low),the switching probability can depend on the width of the write pulse. Inmany embodiments, the write pulse width is approximately equal to halfof the resonant period. Such a configuration can ensure that the devicesuccessfully changes state. The magnetic moment becomes stabilized andaligned with its easy axis via damping and weak precessional motionright after the removal of the applied voltage pulse. Typically, therequired time to make the device stable is a function of the magnitudeof the PMA at the end of the write pulse. Since the device is stillweakly precessing at the end of the write process (subsequent to removalof the voltage pulse), the device can be vulnerable to noise, such asbut not limited to thermal noise. This effect can cause the MEJ toundergo undesired switching if the PMA at zero bias is not high enough,which requires more time to stabilize the magnetic moment via the weakprecessional motion. Furthermore, subsequent write attempts canexperience a different write probability if the oscillations of theresistance (even after a successful write event) have not yet fully beendamped out.

FIGS. 3A and 3B conceptually illustrate the oscillations of theresistance of an MTJ after the removal of an applied voltage inaccordance with an embodiment of the invention. FIG. 3A is a graphillustrating an applied voltage across an MTJ with respect to time. Asshown, a voltage pulse is applied across the MTJ at time t_(0_a), whichcan allow for switching. The voltage pulse is then removed at timet_(1_a). FIG. 3B is a graph showing the MTJ resistance as a function oftime during the time period shown in FIG. 3A. As shown, the resistanceis still weakly oscillating after the removal of the applied voltage attime t_(1_a). During this period, the device can be vulnerable to noise,which can cause the device to switch back to its previous state.

Write errors can be addressed using a variety of different mechanismsand combinations of such mechanisms. Combination of mechanisms can allowfor a compounded effect in some applications. In many embodiments, writeerrors can be addressed through increasing the PMA at the end of thewrite pulse. In some embodiments, word line pulse (“WLP”) circuitschemes are implemented for reducing WER. In several embodiments, thewriting scheme is implemented to improve the write pulse shape in orderto reduce WER. For example, WLP schemes can create an improved squareshaped write pulse across a voltage-controlled MTJ, which can improveswitching probability, and minimize the area overhead (e.g., driversize). U.S. patent application Ser. No. 15/636,568 (“the '568 patentapplication”) to Hochul Lee discusses a set of WLP circuit techniquesfor reducing WER in MeRAM applications. The disclosure of the U.S.patent application Ser. No. 15/636,568 is hereby incorporated byreference in its entirety.

WLP schemes such as those described above can be implemented to createan improved square shaped write pulse, which is accompanied by animproved pulse slew rate. This pulse sharpening, or edge sharpening,effect can be further refined with RVP schemes. In some embodiments, anRVP scheme is implemented by applying a voltage pulse of a givenpolarity across the MEJ bit, which can reduce the perpendicular magneticanisotropy (“PMA”) and the magnetic coercivity of the free layer of theMEJ and cause the magnetization of the free layer of the MEJ to changedirection. A voltage of opposite polarity can be applied across thedevice at the end of the first voltage pulse. This RVP scheme canincrease the falling slope of the write voltage across the MEJ. Sinceprecessional movement does not stop immediately at the end of the writepulse, a pulse of the opposite polarity applied on the MEJ for a shortduration after removing the previously applied positive pulse can helpsecure the written state and avoid undesired switching by increasing PMAafter the write pulse. As a result, WER can be reduced.

Due to the resulting lower WER, the RVP scheme can reduce the totalnumber of write attempts to achieve the same bit error rate (“BER”). TheWER can depend on the timing and amplitude of the pulse as well as therising and falling slope of the pulse. If the WER is high, multiplewrite attempts are required to decrease the total error rate of a bit.Each write attempt reduces the number of errors in a bit by the WER,resulting in a BER that is equal to the WER to the power of writeattempts (BER=WER^(WRITE ATTEMPTS)). Different applications can requiredifferent BER. In embedded memory applications, a BER of 10⁻⁹ istypically desirable. RVP schemes and other writing schemes along withfundamental MEJ structures and their operating principles are discussedbelow in further detail.

Fundamental Magnetoelectric Junction Structures

A fundamental MEJ structure typically includes a ferromagnetic (“FM”)fixed layer, a FM free layer that has a uniaxial anisotropy (forsimplicity, the terms “FM fixed layer” and “fixed layer” will beconsidered equivalent throughout this application, unless otherwisestated; similarly, the terms “FM free layer”, “ferromagnetic freelayer,” “free layer that has a uniaxial anisotropy”, and “free layer”will also be considered equivalent throughout this application, unlessotherwise stated), and a dielectric layer separating the FM fixed layerand FM free layer. Generally, the FM fixed layer has a fixedmagnetization direction—i.e., the direction of magnetization of the FMfixed layer does not change during the normal operation of the MEJ.Conversely, the FM free layer can adopt a magnetization direction thatis either substantially parallel with or antiparallel with the FM fixedlayer—i.e., during the normal operation of the MEJ, the direction ofmagnetization can be made to change. For example, the FM free layer mayhave a magnetic uniaxial anisotropy, whereby it has an easy axis that issubstantially aligned with the direction of magnetization of the FMfixed layer. The easy axis refers to the axis, along which themagnetization direction of the layer prefers to align. In other words,an easy axis is an energetically favorable direction (axis) ofspontaneous magnetization that is determined by various sources ofmagnetic anisotropy including, but not limited to, magnetocrystallineanisotropy, magnetoelastic anisotropy, geometric shape of the layer,etc. Relatedly, an easy plane is a plane whereby the direction ofmagnetization is favored to be within the plane, although there is nobias toward a particular axis within the plane. The easy axis and thedirection of the magnetization of the fixed layer can be considered tobe ‘substantially aligned’ when—in the case where the magnetizationdirection of the free layer conforms to the easy axis—the underlyingprinciples of magnetoresistance take effect and result in a distinctmeasurable difference in the resistance of the MEJ as between when themagnetization directions of the FM layers are substantially parallelrelative to when they are substantially antiparallel, e.g. such that twodistinct information states can be defined. Similarly, the magnetizationdirections of the fixed layer and the free layer can be considered to besubstantially parallel/antiparallel when the underlying principles ofmagnetoresistance take effect and result in a distinct measurabledifference in the resistance of the MEJ as between the two states (i.e.,substantially parallel and substantially antiparallel).

VCMA phenomena can be relied on in switching the FM free layer'scharacteristic magnetization direction—i.e., the MEJ can be configuredsuch that the application of a potential difference across the MEJ canreduce the coercivity of the free layer, which can allow the freelayer's magnetization direction to be switched more easily. For example,with a reduced coercivity, the FM free layer can be subject tomagnetization that can make it substantially parallel with orsubstantially antiparallel with the direction of the magnetization forthe FM fixed layer. A more involved discussion regarding the generaloperating principles of an MEJ is presented in the following section.

Notably, the magnetization direction, and the related characteristics ofmagnetic anisotropy, can be established for the FM fixed and FM freelayers using any suitable method. For instance, the shapes of theconstituent FM fixed layer, FM free layer, and dielectric layer, can beselected based on desired magnetization direction orientations. Forexample, implementing FM fixed, FM free, and dielectric layers that havean elongated shape (e.g., an elliptical cross-section) may tend toinduce magnetic anisotropy that is in the direction of the length of theelongated axis—i.e., the FM fixed and FM free layers will possess atendency to adopt a direction of magnetization along the length of theelongated axis. In other words, the direction of the magnetization is‘in-plane’. Alternatively, where it is desired that the magneticanisotropy have a directional component that is perpendicular to the FMfixed and FM free layers (i.e., ‘out-of-plane’), the shape of the layerscan be made to be symmetrical, e.g. circular, and further the FM layerscan be made to be thin. In this case, while the tendency of themagnetization to remain in-plane may still exist, it may not have apreferred directionality within the plane of the layer. Because the FMlayers are relatively thinner, the anisotropic effects that result frominterfaces between the FM layers and any adjacent layers, which tend tobe out-of-plane, may tend to dominate the overall anisotropy of the FMlayer. Alternatively, a material may be used for the FM fixed or freelayers which has a bulk perpendicular anisotropy—i.e., an anisotropyoriginating from its bulk (volume) rather than from its interfaces withother adjacent layers. The FM free or fixed layers may also consist of anumber of sub-layers, with the interfacial anisotropy between individualsub-layers giving rise to an effective bulk anisotropy to the materialas a whole. Additionally, FM free or fixed layers may be constructedwhich combine these effects, and for example have both interfacial andbulk contributions to perpendicular anisotropy. Of course, any suitablemethods for imposing magnetic anisotropy can be implemented inaccordance with many embodiments of the invention.

While MEJs demonstrate much promise, their potential applicationscontinue to be explored. For example, U.S. Pat. No. 8,841,739 (the '739patent) to Khalili Amiri et al. discloses DIOMEJ cells that utilizediodes (e.g. as opposed to transistors) as access devices to MEJs. Asdiscussed in the '739 patent, using diodes as access devices for MEJscan confer a number of advantages and thereby make the implementation ofMEJs much more practicable. The disclosure of the '739 patent is herebyincorporated by reference in its entirety, especially as it pertains toimplementing diodes as access devices for MEJs. Furthermore, U.S. Pat.No. 9,099,641 (“the '641 patent”) to Khalili Amiri et al. discloses MEJconfigurations that demonstrate improved writeability and readability,and further make the implementation of MEJs more practicable. Thedisclosure of the '641 patent is hereby incorporated by reference in itsentirety, especially as it pertains to MEJ configurations thatdemonstrate improved writeability and readability. Additionally, U.S.patent application Ser. No. 14/681,358 (“the '358 patent application”)to Qi Hu discloses implementing MEJ configurations that incorporatepiezoelectric materials to strain the respective MEJs during operation,and thereby improve performance. The disclosure of the '358 patentapplication is hereby incorporated by reference in its entirety,especially as it pertains to MEJ configurations that incorporateelements configured to strain the respective MEJs during operation, andthereby improve performance.

FIG. 4A conceptually illustrates an MEJ whereby the FM fixed layer andthe FM free layer are separated by, and directly adjoined to, adielectric layer. In particular, in the illustration, the MEJ 400includes an FM fixed layer 402 that is adjoined to a dielectric layer406, thereby forming a first interface 408; the MEJ further includes anFM free layer 404 that is adjoined to the dielectric layer 406 on anopposing side of the first interface 408, and thereby forms a secondinterface 410. The MEJ 400 has an FM fixed layer 402 that has amagnetization direction 412 that is in-plane, and depicted in theillustration as being from left to right. Accordingly, the FM free layeris configured such that it can adopt a magnetization direction 414 thatis either parallel with or antiparallel with the magnetization directionof the FM fixed layer. For reference, the easy axis 416 is illustrated,as well as a parallel magnetization direction 418, and an antiparallelmagnetization direction 420. Additional contacts (capping or seedmaterials, or multilayers of materials, not shown in FIG. 4A) may beattached to the FM free layer 404 and the FM fixed layer 402, therebyforming additional interfaces. FIG. 4B conceptually illustrates an MEJand depicts the constituent cap/seed layers. The contacts can bothcontribute to the electrical and magnetic characteristics of the deviceby providing additional interfaces, and can also be used to apply apotential difference across the device. Additionally, it should ofcourse be understood that MEJs can include metallic contacts that canallow them to interconnect with other electrical components.

Importantly, by appropriately selecting the materials, the MEJ can beconfigured such that the application of a potential difference acrossthe FM fixed layer and the FM free layer can modify the magneticanisotropy, and correspondingly reduce the coercivity, of the FM freelayer. For example, whereas in FIGS. 4A and 4B, the magnetizationdirection of the FM free layer is depicted as being in-plane, theapplication of a voltage may distort the magnetization direction of theFM free layer such that it includes a component that is at leastpartially out of plane. The particular dynamics of the modification ofthe magnetic anisotropy will be discussed below in the section entitled“General Principles of MEJ Operation.” Suitable materials for the FMlayers such that this effect can be implemented include iron, nickel,manganese, cobalt, FeCoB, FeGaB, FePd, FePt; further, any compounds oralloys that include these materials may also be suitable. Suitablematerials for the dielectric layer include MgO and Al₂O₃. Of course, itshould be understood that the material selection is not limited to thoserecited—any suitable FM material can be used for the FM fixed and freelayers and any suitable material can be used for the dielectric layer.It should also be understood that each of the FM free layer, FM fixedlayer, and dielectric layer may consist of a number of sub-layers, whichacting together provide the functionality of the respective layer.

FIG. 5 conceptually illustrates an MEJ whereby the orientation of themagnetization directions is perpendicular to the plane of theconstituent layers (“perpendicular magnetic anisotropy”). In particular,the MEJ 500 is similarly configured to that seen in FIG. 4A, with an FMfixed layer 502 and an FM free layer 504 adjoined to a dielectric layer506. However, unlike the MEJ in FIG. 4A, the magnetization directions ofthe FM fixed and FM free layers, 508 and 510 respectively, are orientedperpendicularly to the layers of the MEJ. Additional contacts (cappingor seed materials, or multilayers of materials, not shown) may beattached to the FM free layer 504 and the FM fixed layer 502, therebyforming additional interfaces. The contacts both contribute to theelectrical and magnetic characteristics of the device by providingadditional interfaces, and can also be used to apply a potentialdifference across the device. It should also be understood that each ofthe FM free layer, FM fixed layer, and dielectric layer may consist of anumber of sub-layers, which acting together provide the functionality ofthe respective layer.

Of course, it should be understood that the direction of magnetizationfor the FM layers can be in any direction, as long as the FM free layercan adopt a direction of magnetization that is either substantiallyparallel with or antiparallel with the direction of magnetization of theFM fixed layer. For example, the direction of magnetization can includeboth in-plane and out-of-plane components.

In many instances, an MEJ includes additional adjunct layers thatfunction to facilitate the operation of the MEJ. For example, in manyinstances, the FM free layer includes a capping or seed layer, which can(1) help induce greater electron spin perpendicular to the surface ofthe layer, thereby increasing its perpendicular magnetic anisotropy,and/or (2) can further enhance the sensitivity to the application of anelectrical potential difference. In general, the seed/cap layers canbeneficially promote the crystallinity of the ferromagnetic layers. Theseed layer can also serve to separate a corresponding ferromagneticlayer from an ‘underlayer.’ As will be discussed below, in manyembodiments of the invention, the capping/seed layer includes one of:Hf, Mo, W, Ir, Bi, Re, and/or Au; the listed elements can beincorporated by themselves, in combination with one another, or incombination with more conventional materials, such as Ta, Ru, Pt, Pd. Aswill be discussed in greater detail below, seed and/or cap layers madein this way can confer a number of benefits to the MEJ structure.

FIG. 6 conceptually illustrates an MEJ 600 that includes multiple layersthat work in aggregate to facilitate the functionality of the MEJ 600. Apillar section 602 extends from a planar section 604. A voltage is shownbeing applied 606 between the top and bottom of the pillar. By way ofexample, the planar section 604 includes an Si/SiO₂ substrate 608adjoined to a bottom electrode 610. In the illustrated embodiment, thepillar 602 includes the following layers in order: Ta 612 (e.g., 5 nm inthickness); a free layer 614 having an Fe-rich CoFeB material (e.g.,Co₂₀Fe₆₀B₂₀ having a thickness generally ranging from, but not limitedto, 0.8 nm-1.6 nm); a dielectric layer 616 having a dielectric oxidesuch as MgO or Al₂O₃ having a thickness of approximately, but notlimited to, 0.8-1.4 nm); a FM fixed layer 618 having a CoFeB material(e.g., Co₆₀Fe₂₀B₂₀ having a thickness of approximately, but not limitedto, 2.7 nm); a metal layer (e.g. Ru 620 having a thickness ofapproximately, but not limited to, 0.85 nm) to provide antiferromagneticinter-layer exchange coupling; an exchange-biased layer 622 of Co₇₀Fe₃₀(e.g., thickness of approximately, but not limited to, 2.3 nm), themagnetization orientation of which is pinned by exchange bias using ananti-ferromagnetic layer 624 (e.g., PtMn, IrMn, or a like materialhaving a thickness of approximately, but not limited to, 20 nm); and atop electrode 626. By way of example and not limitation, the pillar ofthe device depicted is in the shape of a 170 nm×60 nm ellipticalnanopillar. In this illustration, Ta layer 612 is used as a seed layerto help induce a larger magnitude of perpendicular magnetic anisotropyand/or enhance the electric-field sensitivity of magnetic properties(such as anisotropy) in the FM free layer. It also acts as a sink of Batoms during annealing of the material stack after deposition, resultingin better crystallization of the FM free layer and thereby increasingthe TMR and/or VCMA effect. Of course, any suitable materials can beused as a capping or seed layer 612; for example, as will be discussedin greater detail below, in many embodiments of the invention, the seedand/or cap layers include one of: Mo, W, Hf, Ir, Bi, Rh, and/or Au. Moregenerally, any adjunct layers that can help facilitate the properfunctioning of the MEJ can be implemented in an MEJ.

MEJs can also include a semi-fixed layer, which has a magneticanisotropy that is altered by the application of a potential difference.In many instances, the characteristic magnetic anisotropy of thesemi-fixed layer is a function of the applied voltage. For example, inmany cases, the direction of the magnetization of the semi-fixed layeris oriented in the plane of the layer in the absence of a potentialdifference across the MEJ. However, when a potential difference isapplied, the magnetic anisotropy is altered such that the magnetizationincludes a strengthened out-of-plane component. Moreover, the extent towhich the magnetic anisotropy of the semi-fixed layer is modified as afunction of applied voltage can be made to be less than the extent towhich the magnetic anisotropy of the FM free layer is modified as afunction of applied voltage. The incorporation of a semi-fixed layer canfacilitate a more nuanced operation of the MEJ (to be discussed below inthe section entitled “General Principles of MEJ Operation”).

FIG. 7A conceptually illustrates an MEJ that includes a semi-fixed layerin accordance with an embodiment of the invention. In particular, theconfiguration of the MEJ 700 is similar to that depicted in FIG. 4A,insofar as it includes an FM fixed layer 702 and an FM free layer 704separated by a dielectric layer 706. However, the MEJ 700 furtherincludes a second dielectric layer 708 adjoined to the FM free layer 704such that the FM free layer 704 is adjoined to two dielectric layers,706 and 708 respectively, on opposing sides. Further, a semi-fixed layer710 is adjoined to the dielectric layer. Typically, the direction ofmagnetization of the semi-fixed layer 714 is antiparallel with that ofthe FM fixed layer 712. As mentioned above, the direction ofmagnetization of the semi-fixed layer can be manipulated based on theapplication of a voltage. In the illustration, it is depicted that theapplication of a potential difference adjusts the magnetic anisotropy ofthe semi-fixed layer such that the strength of the magnetization along adirection orthogonal to the initial direction of magnetization (in thiscase, out of the plane of the layer) is developed. It should of coursebe noted that the application of a potential difference can augment themagnetic anisotropy in any number of ways; for instance, in some MEJs,the application of a potential difference can reduce the strength of themagnetization in a direction orthogonal to the initial direction ofmagnetization. Note also that in the illustration, the directions ofmagnetization are all depicted to be in-plane where there is nopotential difference. However, of course it should be understood thatthe direction of the magnetization can be in any suitable direction.More generally, although a particular configuration of an MEJ thatincludes a semi-fixed layer is depicted, it should of course beunderstood that a semi-fixed layer can be incorporated within an MEJ inany number of configurations. For example, FIG. 7B conceptuallyillustrates an MEJ that includes a semi-fixed layer that is in adifferent configuration than that seen in 7A. In particular, the MEJ 750is similar to that seen in FIG. 7A, except that the positioning of thesemi-fixed layer 752 and the free layer 754 is inverted. In certainsituations, such a configuration may be more desirable. The generalprinciples of the operation of an MEJ are now discussed.

General Principles of MEJ Operation

MEJ operating principles—as they are currently understood—are nowdiscussed. Note that embodiments of the invention are not constrained tothe particular realization of these phenomena. Rather, the presumedunderlying physical phenomena is being presented to inform the reader asto how MEJs are believed to operate. MEJs generally function to achievetwo distinct states using the principles of magnetoresistance. Asmentioned above, magnetoresistance principles regard how the resistanceof a thin film structure that includes alternating layers offerromagnetic and non-magnetic layers depends upon whether theferromagnetic layers are in a substantially parallel or antiparallelalignment. Thus, an MEJ can achieve a first state where its FM layershave magnetization directions that are substantially parallel, and asecond state where its FM layers have magnetization directions that aresubstantially antiparallel. MEJs further rely on voltage-controlledmagnetic anisotropy phenomena. Generally, VCMA phenomena regard how theapplication of a voltage to a ferromagnetic material that is adjoined toan adjacent dielectric layer can impact the characteristics of theferromagnetic material's magnetic anisotropy. For example, it has beendemonstrated that the interface of oxides such as MgO with metallicferromagnets such as Fe, CoFe, and CoFeB can exhibit a largeperpendicular magnetic anisotropy which is furthermore sensitive tovoltages applied across the dielectric layer, an effect that has beenattributed to spin-dependent charge screening, hybridization of atomicorbitals at the interface, and to the electric field induced modulationof the relative occupancy of atomic orbitals at the interface. MEJs canexploit this phenomenon to achieve two distinct states. For example,MEJs can employ one of two mechanisms to do so: first, MEJs can beconfigured such that the application of a potential difference acrossthe MEJ functions to reduce the coercivity of the FM free layer, suchthat it can be subject to magnetization in a desired magnetic direction,e.g. either substantially parallel with or antiparallel with themagnetization direction of the fixed layer; second, MEJ operation canrely on precessional switching (or resonant switching), whereby byprecisely subjecting the MEJ to voltage pulses of precise duration, thedirection of magnetization of the FM free layer can be made to switch.

In many instances, MEJ operation is based on reducing the coercivity ofthe FM free layer such that it can adopt a desired magnetizationdirection. With a reduced coercivity, the FM free layer can adopt amagnetization direction in any suitable way. For instance, themagnetization can result from: an externally applied magnetic field, themagnetic field of the FM fixed layer; the application of a spin-transfertorque (“STT”) current; the magnetic field of a FM semi-fixed layer; theapplication of a current in an adjacent metal line inducing a spin-orbittorque (“SOT”); and any combination of these mechanisms, or any othersuitable method of magnetizing the FM free layer with a reducedcoercivity.

By way of example and not limitation, examples of suitable ranges forthe externally applied magnetic field are in the range of 0 to 100 Oe.The magnitude of the electric field applied across the device to reduceits coercivity or bring about resonant switching can be approximately inthe range of 0.1-2.0 V/nm, with lower electric fields required formaterials combinations that exhibit a larger VCMA effect. The magnitudeof the STT current used to assist the switching may be in the range ofapproximately 0.1-1.0 MA/cm².

FIG. 8A depicts how the application of a potential difference can reducethe coercivity of the free layer such that an externally appliedmagnetic field H can impose a magnetization switching on the free layer.In the illustration, in step 1, the FM free layer and the FM fixed layerhave a magnetization direction that is substantially in plane; the FMfree layer has a magnetization direction that is parallel with that ofthe FM fixed layer. Further, in Step 1, the coercivity of the FM freelayer is such that the FM free layer is not prone to having itsmagnetization direction reversed by the magnetic field H, which is in adirection antiparallel with the magnetization direction of the FM fixedlayer. However, a voltage V_(c) is then applied, which results in step2, where the voltage V_(c) has magnified the perpendicular magnetizationdirection component of the free layer (out of its plane).Correspondingly, the coercivity of the FM free layer is reduced suchthat it is subject to magnetization by an in-plane magnetic field H.Accordingly, when the potential difference V_(c) is removed, VCMAeffects are removed and the magnetic field H, which is substantiallyanti-parallel to the magnetization direction of the FM fixed layer,causes the FM free layer to adopt a direction of magnetization that isantiparallel with the magnetization direction of the FM fixed layer.Hence, as the MEJ now includes an FM fixed layer and an FM free layerthat have magnetization directions that are antiparallel, it reads out asecond information state (resistance value) different from the first. Ingeneral, it should be understood that in many embodiments where themagnetization directions of the free layer and the fixed layer aresubstantially in-plane, the application of a voltage enhances theperpendicular magnetic anisotropy such that the FM free layer can becaused to adopt an out-of-plane direction of magnetization. Stateddifferently, the magnetoelectric junction is configured such that when apotential difference is applied across the magnetoelectric junction, themagnetic anisotropy of the ferromagnetic free layer is altered such thatthe relative strength of the magnetic anisotropy along a second easyaxis that is orthogonal to the first easy axis (which corresponds to themagnetization direction of the fixed layer), or the easy plane wherethere is no easy axis that is orthogonal to the first easy axis, ascompared to the strength of the magnetic anisotropy along the first easyaxis, is magnified or reduced for the duration of the application of thepotential difference. The magnetization direction can thereby be made toswitch. In general, it can be seen that by controlling the potentialdifference and the direction of an applied external magnetic field, anMEJ switch can be achieved.

It should of course be understood that the direction of the FM fixedlayer's magnetization direction need not be in-plane—it can be in anysuitable direction. For instance, it can be substantially out of plane.Additionally, the FM free layer can include both in-plane andout-of-plane magnetic anisotropy directional components. FIG. 8B depictsa corresponding case relative to FIG. 8A when the FM fixed and FM freelayers have magnetization directions that are perpendicular to thelayers of the MEJ (out-of-plane). It is of course important, that an FM,magnetically anisotropic, free layer be able to adopt a magnetizationdirection that is either substantially parallel with an FM fixed layer,or substantially antiparallel with an FM fixed layer. In other words,when unburdened by a potential difference, the FM free layer can adopt adirection of magnetization that is either substantially parallel with orantiparallel with the direction of the FM fixed layer's magnetization,to the extent that a distinct measurable difference in the resistance ofthe MEJ that results from the principles of magnetoresistance as betweenthe two states (i.e., parallel alignment vs. antiparallel alignment) canbe measured, such that two distinct information states can be defined.

Note of course that the application of an externally applied magneticfield is not the only way for the MEJ to take advantage of reducedcoercivity upon application of a potential difference. For example, themagnetization of the FM fixed layer can be used to impose amagnetization direction on the free layer when the free layer has areduced coercivity. Moreover, an MEJ can be configured to receive aspin-transfer torque current when application of a voltage causes areduction in the coercivity of the FM free layer. Generally, STT currentis a spin-polarized current that can be used to facilitate the change ofmagnetization direction on a ferromagnetic layer. It can originate, forexample, from a current passed directly through the MEJ device, such asdue to leakage when a voltage is applied, or it can be created by othermeans, such as by spin-orbit-torques (e.g., Rashba or Spin-Hall Effects)when a current is passed along a metal line placed adjacent to the FMfree layer. Accordingly, the spin orbit torque current can then helpcause the FM free layer to adopt a particular magnetization direction,where the direction of the spin orbit torque determines the direction ofmagnetization. This configuration is advantageous over conventionalSTT-RAM configurations since the reduced coercivity of the FM free layerreduces the amount of current required to cause the FM free layer toadopt a particular magnetization direction, thereby making the devicemore energy efficient.

FIG. 9 depicts using a metal line disposed adjacent to an FM free layerto generate spin-orbit torques that can impose a magnetization directionchange on the FM free layer. In particular, the MEJ 900 is similar tothat seen in FIG. 4A, except that it further includes a metal line 902,whereby a current 904 can flow to induce spin-orbit torques, which canthereby help impose a magnetization direction change on theferromagnetic free layer.

Additionally, in many instances, an MEJ cell can further take advantageof thermally assisted switching (“TAS”) principles. Generally, inaccordance with TAS principles, heating up the MEJ during a writingprocess reduces the magnetic field required to induce switching. Thus,for instance, where STT is employed, even less current may be requiredto help impose a magnetization direction change on a free layer,particularly where VCMA principles have been utilized to reduce itscoercivity.

Moreover, the switching of MEJs to achieve two information states canalso be achieved using voltage pulses. In particular, if voltage pulsesare imposed on the MEJ for a time period that is one-half of theprecession of the magnetization of the free layer, then themagnetization may invert its direction. Using this technique, ultrafastswitching times (e.g., below 1 ns) can be realized; moreover, usingvoltage pulses as opposed to a current, makes this technique moreenergetically efficient as compared to the precessional switchinginduced by STT currents, as is often used in STT-RAM. However, thistechnique is subject to the application of a precise pulse that is halfthe length of the precessional period of the magnetization layer. Theprecessional period of the magnetization layer can be defined as thetime it takes for the magnetization to undergo one cycle of oscillation.For instance, it has been observed that pulse durations in the range of0.05 to 3 nanoseconds can reverse the magnetization direction.Additionally, the voltage pulse must be of suitable amplitude to causethe desired effect—e.g., reverse the direction of magnetization.

Based on this background, it can be seen that MEJs can confer numerousadvantages relative to conventional MTJs. For example, they can becontrolled using voltages of a single polarity—indeed, the '739 patent,incorporated by reference above, discusses using diodes, in lieu oftransistors, as access devices to the MEJ, and this configuration isenabled because MEJs can be controlled using voltage sources of a singlepolarity.

Note that while the above discussion largely regards the operation ofsingle MEJs, it should of course be understood that in many instances, aplurality of MEJs are implemented together. For example, the '671 patentapplication discloses MeRAM configurations that include a plurality ofMEJs disposed in a cross-bar architecture. It should be clear that MEJsystems can include a plurality of MEJs in accordance with embodimentsof the invention. Where multiple MEJs are implemented, they can beseparated by field insulation, and encapsulated by top and bottomlayers. Thus, for example, FIG. 10 depicts the implementation of twoMEJs that are housed within encapsulating layers and separated by fieldinsulation. In particular, the MEJs 1002 are encapsulated within abottom layer 1004 and a top layer 1006. Field insulation 1008 isimplemented to isolate the MEJs and facilitate their respectiveoperation. It should of course be appreciated that each of the top andbottom layers can include one or multiple layers ofmaterials/structures. As can also be appreciated, the field insulationmaterial can be any suitable material that functions to facilitate theoperation of the MEJs in accordance with embodiments of the invention.While a certain configuration for the implementation of a plurality ofMEJs has been illustrated and discussed, any suitable configuration thatintegrates a plurality of MEJs can be implemented in accordance withembodiments of the invention.

RVP Writing Schemes

Various writing schemes can be implemented in MeRAM applications. Inmany embodiments, writing schemes are implemented to reduce WER in MeRAMapplications. Write error in MeRAM is mainly caused by a degraded writepulse (e.g., slew rate and duration), and can limit its applications inhigh-speed memories. If WER is relatively high, multiple writeoperations are required to achieve an acceptable BER. As such, byreducing WER, less write operations are required for a given BER.

FIG. 11 conceptually illustrates a MeRAM bit-cell in accordance with anembodiment of the invention. As shown, the MeRAM cell 1100 includes anMEJ 1102 and an access transistor 1104. The MEJ 1 includes at least thefollowing layers: a pinned layer (“PL”) 1106; a tunnel barrier (“TB”)1108; and a magnetic free layer (“FL”) 1110. The two terminals of theMEJ can be referred to as the top electrode (“TE”) 1112 and the bottomelectrode (“BE”) 1114, respectively. From the bit-cell point of view,there are three electrical ports to read and write an MEJ: a bit line(“BL”) 1116; a word line (“WL”) 1118; and a source line (“SL”) 1120. Inthe illustrated embodiment, the MEJ is configured to have a lowresistance when the magnetic moments of the FL and PL are aligned alongthe same direction—i.e., in the parallel state (denoted as P). In theanti-parallel state (denoted as AP), the FL magnetization is aligned inthe opposite direction to the PL, resulting in a high resistance.Although the above description refers to a structure with the FL on thebottom and other layers in a prescribed order, this ordering can varyfrom application to application. In a number of embodiments, thisordering is reversed. Depending on the specific ordering, the operationof the device can differ accordingly.

Conventional writing schemes in MeRAM applications can include applyinga voltage pulse to the WL or the BL in order to generate the write pulseacross the BE to the TE, which can decrease the PMA of the MEJ andallows precessional switching between the two states. The VCMA effectcan modulate the PMA (hence free layer coercivity) under the electricbias condition (as shown in FIG. 1). It has been shown that a negative(positive) voltage across the perpendicularly magnetized MEJ increases(decreases) the coercivity of the free layer. By allowing the MEJ toprecess, the MEJ can switch to the opposite state (as shown in FIG. 2).

In many embodiments, a reverse pulse scheme can be implemented in aMeRAM application to reduce WER. In some embodiments, the writingmechanism is implemented for a voltage controlled magnetoelectric tunneljunction, or MEJ, with an in-plane or perpendicular magnetization thatis implemented as the storage element in a MeRAM cell. A reverse pulsescheme can be implemented with a write operation that includes a writevoltage pulse along with a reverse pulse that increases the PMA at theend of the write pulse. For example, a write operation can beimplemented by applying a voltage pulse of a given polarity across theMEJ bit, which can reduce the perpendicular magnetic anisotropy (“PMA”)and the magnetic coercivity of the free layer of the MEJ and cause themagnetization of the free layer of the MEJ to change direction. Avoltage of opposite polarity can be applied across the device at the endof the first voltage pulse. This RVP scheme can increase the fallingslope of the write voltage across the MEJ. In a number of embodiments, apulse of a polarity opposite the write pulse can be applied before thewrite operation to stabilize the bit state before the write operation.

To achieve low WER, the PMA can be modulated instantly to have a stableprecessional trajectory. In a number of embodiments, the device isconfigured such that rising/falling edges of the applied voltage are assharp as possible. Since precessional movement does not stop immediatelyat the end of the write pulse (as shown in FIGS. 3A and 3B), a negativepulse (or a pulse of the opposite polarity from the BE to the TE) can beapplied on the MEJ for a short duration after removing the previouslyapplied positive pulse, which can enhance coercivity to secure thewritten state and avoid undesired switching. The scheme as describedabove can be important to VCMA-driven precessional switching, and maynot be as critical in conventional methods used for switchingmagnetization in magnetic tunnel junctions, such as thermally activatedswitching driven by current and/or magnetic fields, since the timescales of such switching events are much longer than the oscillations ofthe magnetization considered in devices in accordance with variousembodiments of the invention.

In many embodiments, the reverse pulse scheme utilizes the VCMA effectin reverse to increase the rising/falling slope(s), which can improveWER by engineering the control signals of the MeRAM. FIG. 12conceptually illustrates a circuit diagram of MeRAM cells in accordancewith an embodiment of the invention. In the illustrated embodiment, theWL Driver 1202, BL Driver 1204, and SL Driver 1206 are coupled to the WL1212, the BL 1214, and the SL 1216, respectively. A number of MeRAMcells can share the BL and SL in the vertical direction, and anothernumber of cells can share the WL in the horizontal direction to createan array of MeRAM cells. The controls can be engineered such that thewrite pulse, generated from the pulse generator 1208, is given from BL,SL, WL, or a combination of either (e.g., WL rise as the rising edge ofthe write pulse and BL fall as the falling edge of the write pulse).V_(SL), V_(WL), V_(BL), and V_(MEJ) describe the voltage at theirrespective component.

The reverse pulse can be applied using a variety of different methods.In many embodiments, the reverse pulse can be applied using any of theelectrical ports of the MeRAM cells (i.e., WL, BL, or SL). FIGS. 13A-13Cconceptually illustrate three examples of applying a reverse voltage onthe MEJ after the write pulse in accordance with various embodiments ofthe invention. In the illustrated embodiments, a time graph of thevoltages V_(MEJ), V_(WL), V_(BL), and V_(SL) are shown. The voltageacross the MEJ (V_(MEJ)) is equal to the voltage at TE minus the voltageat BE (V_(MEJ)=V_(TE)−V_(BE)). In FIG. 13A, the reverse pulse isimplemented by applying a voltage pulse to the SL at the end of thewrite pulse on the BL such that the voltage across the MEJ is ofopposite polarity. In many embodiments, the rising edge of the SLcoincides with the falling edge of the BL. The falling edge slope of theBL and the rising slope of the SL can add on the differential nodes ofthe device, which can increase the falling edge slope. As such, thefalling edge slope on the device is greater than that of the fallingedge slope on the BL. This method can be applied without the need forany boosted voltage, regulated voltage, or negative voltages. FIG. 13Bshows applying a reverse voltage on the MEJ using the falling edge ofthe WL to capacitively couple to the nodes of the MEJ through thegate-to-source-capacitance. The coupling can create an instantaneousnegative potential on the device, which then slowly leaks back to theground potential. The WL can be decreased to below ground potential toincrease the amount of coupling. In FIG. 13C, the reverse pulse isimplemented by applying a negative voltage on the BL at the end of thewrite pulse. In some embodiments, the negative voltage is applied on theBL while the SL is kept at ground level. In other embodiments, thevoltage level of the SL and the voltage applied on the BL are bothpositive with respect to ground. Although FIGS. 13A-13C illustrate threespecific methods of applying a reverse pulse, any of a number of methodscan be utilized to apply a reverse voltage on the MEJ after the writepulse. The combined application of two or more of the RVPimplementations can provide additional gain in the effectiveness of suchRVP schemes. For example, using the WL coupling in conjunction with SLpulse can provide a further increase in the falling slope as well as thelevel of the reverse pulse.

Although specific reverse pulse schemes for MeRAM applications arediscussed above, a person having ordinary skill in the art wouldappreciate that any of a number of reverse pulse writing schemes can beimplemented in accordance with embodiments of the invention. It istherefore to be understood that the present invention may be practicedin ways other than specifically described, without departing from thescope and spirit of the present invention. Thus, embodiments of thepresent invention should be considered in all respects as illustrativeand not restrictive. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated, but by the appendedclaims and their equivalents.

Optimization of Magnetic Torque

In addition to optimizing the magnetic trajectory (as discussed above),optimization of magnetic torque can also be utilized to enhance theperformance of the device. In many embodiments, it is desirable that theprecessional dynamics are cutoff at the maximum of the magnetictrajectory, m_(z,max). As discussed above, this can be achieved by asharp and strong increase in anisotropy at half of the precessionalperiod, t_(precess), which can be implemented through either a high slewrate or a negative voltage. However, the instantaneous change ofanisotropy can result in large residual torque and, therefore, ringingof the magnetization after the write operation, which can make thedevice susceptible to noise and can result in undesirable switching(write errors). As such, the final torque at the end of the pulse duringthe write operation can greatly impact writing errors. In many cases,reducing the final torque can be important for lowering WER, whilereducing initial torque can impact stochastic signal generation.Compared to trajectory optimization, torque optimization can achievesimilar error rates with an additional ˜43% improvement in ringing and2.1× improvement in pulse variation tolerance. In addition, a ˜37%improvement in energy and ˜38% improvement in delay can be achieved forstochastic signal generation.

In many embodiments, the magnetic torque at m_(z,max) is optimized tominimize ringing. In some cases, this translates to minimizing themagnetic torque at m_(z,max). In several embodiments, an asymmetricpulse with an early, gradual ramp down of the voltage is utilized tominimize the magnetic torque at m_(z,max). As the voltage slowly reduces(ramp down), the torque decreases and the precession slows down as themagnetization approaches m_(z,max). An asymmetric ramp can be achievedusing various methods, including but not limited to configurable driversizes. In several embodiments, implementation of the asymmetric rampdoes not require complex timing/regulator circuits.

FIGS. 14A-14D conceptually illustrate a comparison of two write methods,including the optimization of magnetic torque, in accordance withvarious embodiments of the invention. FIG. 14A-14C respectively show thevoltage waveform, magnetization, and magnetic torque plots of the twowrite methods. As shown, lines 1400A, 1400B, and 1400C illustrate theprecessional trajectory, where the precession period of the MTJ is 2.3ns. Lines 1401A, 1401B, and 1401C show a method where the pulse isterminated sharply at the maximum magnetization. Lines 1402A, 1402B, and1402C show a write method implementing a ramp down period, resulting inlower torque at the maximum magnetization. FIG. 14D shows a 3Dvisualization of the magnetization. By minimizing torque, the ringing(radius of the circular trajectory 1403 shown in FIG. 14D) can beminimized.

In many embodiments, the magnetic torque can be further optimized byimplementing a two-step ramping process instead of only changing thefalling edge in a single linear step. In further embodiments, the rampincludes two linear steps. The first linear step can include a ramp fromthe write voltage to a predetermined voltage (such as 0.6 V in somecases), and the second step can include a ramp from the predeterminedvoltage to 0V. Two optimized ramps, or steps, for minimal Δm_(z) can bedetermined for each pulse width. FIG. 15 shows optimal ramps fordifferent pulse widths in accordance with various embodiments of theinvention. As shown, Δm_(z) values indicate improvements over singleramp methods.

Although specific methods of implementing optimization of magnetictorque are discussed, many other forms of implementations can beutilized as appropriate to the specific requirements of a givenapplication. For example, in many embodiments, optimization of magnetictorque is implemented using a multi-step ramp, which can include threeor more steps. In some embodiments, optimization of magnetic torque isimplemented along with a reverse pulse scheme. These and other exemplaryforms of implementations are shown in FIGS. 16A-16E. In the illustrativeembodiment, a time graph of the voltages V_(WL), V_(BL), V_(SL), andV_(MTJ) are shown for various implementations, including optimization ofmagnetization (or magnetic trajectory), optimization of magnetic torque,optimization of magnetic torque including a post-RVP scheme,optimization of magnetic torque including a pre-RVP scheme, andoptimization of magnetic torque including a two-step rampdown.

Randomness Simulation Applications

As discussed in the sections above, a MeRAM device can operate as amemory device by switching the device using a pulse timed to half oft_(precess). During such operation, the magnetization precesses for halfof a cycle, resulting in an inversion of the state of the device.Applying the converse of this principle, a MeRAM device can beconfigured to operate as a random number generator by applying a longpulse on the device. With a long pulse, damping can cause the magnetismto eventually align with the in-plane axis, leading to the device to bein a metastable state. When the pulse is removed, thermal noise cancause the metastable state to randomly align with one of the two stablestates. FIGS. 17A and 17B conceptually illustrate the applications ofMeRAM as memory and as a random number generator (RNG) in accordancewith various embodiments of the invention. FIG. 17A shows the voltagewaveforms for the operation of the two applications, while FIG. 17Bshows the Z-magnetization. As shown, the magnetization undergoes adampened oscillation with a period of t_(precess) when a pulse isapplied (solid). In a memory application (dashed), a pulse of halft_(precess) can switch the state of the device. On the other hand, anRNG application (dotted) can be implemented with a long pulse, which canallow the oscillation to dampen and cease. In such cases, the devicestate is random when the voltage is removed.

For RNG applications, the time for generating a stochastic bit candepend on the time it takes for the precession amplitude to fall belowthe noise level. A fast ramp generates a large damping torque, but alsocreates a large initial torque. A slow ramp reduces the initial torque,but also results in smaller damping torque and additional ramp delay. Inmany embodiments, a balance between the two effects can be evaluated todetermine an optimal convergence time. Convergence time can be definedas the time from the start of the pulse to the time that magneticoscillation amplitude is below a threshold m_(th). FIG. 18 conceptuallyillustrates the convergence time for different ramp times and m_(th) of0.1, 0.05, and 0.02 in accordance with various embodiments of theinvention. For m_(th)=0.1, the lowest convergence time occurs at ramptime=10 ns, where the convergence time (40 ns) is 25 ns faster than amagnetic trajectory optimization method (65 ns). Furthermore, the energy(87 fJ) is 37% lower than the magnetic trajectory optimization case (139fJ).

FIG. 19A shows simulations of the probability of P-state as a functionof time for the magnetic trajectory optimization (square) vs. magnetictorque optimization (diamond) in accordance with various embodiments ofthe invention. As shown in FIG. 19A, the probability of the P-stateoscillates and eventually converges to near 50%, characteristic of theprecessional motion. FIG. 19B shows the difference from ideal RNG (Pstate probability of 50%) as a function of the ramp time, sampled with aperiod of 1 ns from pulse width from 70 ns to 80 ns.

Modeling and Simulations

An LLG-based model can be used to evaluate the two methods ofoptimization, trajectory optimization and torque optimization. FIG. 20describes the parameters used in the model. For both memory and RNGapplications, the performance can be measured by first quantifyingdeterministically, and then with stochastic simulations. For memoryoperation, the write error can be quantified by measuring the amount ofnoise that the device can tolerate at its worst-case condition—i.e., theweakest z-magnetization after the write pulse:Δm _(z) ≡m _(z)(t=inf)−min(m _(z)(t=tpulse→inf)

FIG. 21A shows the magnetic torque at m_(z,max) for differentpulse-widths and falling edge ramp times in accordance with anembodiment of the invention. As shown, minimizing the torque results in12.8× lower torque at m_(z,max), compared to the trajectory optimizationmethod. In the illustrative embodiment, the torque is minimized at(pulse width, falling edge)=(1.07 ns, 0.16 ns); while the conventionalcase is (1.14 ns, 0.01 ns). FIG. 21B shows Δm, and the correspondingpoints. As shown, minimizing the torque also minimizes Δm_(z), 43% lowerthan the write operation for the trajectory optimization method. Alinear relation between the pulse width and the ramp time for lowest lowplot Δm_(z) can be observed, related to the precessional period of thedevice.

The WER can then be simulated directly using stochastic LLG equations.FIGS. 22A-22C shows a simulated WER plot and variation tolerance of adevice in accordance with an embodiment of the invention. As shown,optimization of torque gives slightly lower error rates (˜1.6×),compared to the trajectory optimization method. FIG. 22B shows theamount of variation in the pulse shape that can be tolerated for a givenerror rate. As shown, the two methods achieve similar tolerance forpulse width (given a fixed ramp time); however, optimization for torquecan allow for significantly larger margin (2.1×) for ramp variations.

DOCTRINE OF EQUIVALENTS

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof. It istherefore to be understood that the present invention may be practicedin ways other than specifically described, without departing from thescope and spirit of the present invention. Thus, embodiments of thepresent invention should be considered in all respects as illustrativeand not restrictive. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated, but by the appendedclaims and their equivalents.

What is claimed is:
 1. A method for a writing mechanism for amagnetoelectric random access memory cell, the method comprising:applying a voltage of a given polarity for a given period of time acrossa magnetoelectric junction bit of the magnetoelectric random accessmemory cell, wherein: the magnetoelectric junction bit comprises: aferromagnetic free layer, a ferromagnetic fixed layer, and a dielectriclayer interposed between the ferromagnetic free layer and theferromagnetic fixed layer; application of the voltage of the givenpolarity across the magnetoelectric junction bit reduces theperpendicular magnetic anisotropy and magnetic coercivity of theferromagnetic free layer through a voltage controlled magneticanisotropy effect; and the magnetization of the ferromagnetic free layerchanges direction in response to the application of the voltage of thegiven polarity for the given period of time; and lowering the appliedvoltage of the given polarity before the end of the given period oftime, wherein the given period of time is approximately half of aprecessional period of the ferromagnetic free layer.
 2. The method ofclaim 1, wherein: the magnetoelectric random access memory cellcomprises: a first terminal coupled to a bit line, a second terminalcoupled to a source line, and a third terminal coupled to a word line;and the magnetoelectric junction bit is coupled to the drain of an MOStransistor.
 3. The method of claim 1, further comprising applying avoltage of a polarity opposite the given polarity across themagnetoelectric junction bit at the end of the application of thevoltage of the given polarity.
 4. The method of claim 2, wherein thevoltage of the given polarity is applied using a pulse generator.
 5. Themethod of claim 4, wherein the pulse generator is selected from thegroup consisting of a bit line driver, a source line driver, and a wordline driver.
 6. The method of claim 5, wherein: the rising edge of theapplication of the voltage of the given polarity decreases theperpendicular magnetic anisotropy and causes a precessional motion ofmagnetization between two states of the ferromagnetic free layer; themagnetization direction of the ferromagnetic free layer is differentbetween the two states; and the falling edge of the application of thevoltage of the given polarity restores the decrease in the perpendicularmagnetic anisotropy and stops the precessional motion of magnetization.7. The method of claim 6, further comprising applying a voltage of apolarity opposite the given polarity across the magnetoelectric junctionbit at the end of the application of the voltage of the given polarity,wherein the voltage of the polarity opposite the given polarity isapplied across the magnetoelectric junction bit subsequent or nearsimultaneously with the falling edge of the application of the voltageof the given polarity to increase the perpendicular magnetic anisotropyof the ferromagnetic free layer.
 8. The method of claim 7, wherein thevoltage of the polarity opposite the given polarity is applied usingcapacitive coupling from the word line to the magnetoelectric junctionbit through the gate-to-source-capacitance.
 9. The method of claim 7,wherein the voltage of the polarity opposite the given polarity isapplied through generating a negative voltage with respect to ground onthe bit line while keeping a voltage of the source line at ground level.10. The method of claim 7, wherein the voltage of the polarity oppositethe given polarity is applied by generating a positive voltage pulse onthe source line after a write voltage pulse on the bit line.
 11. Amagnetoelectric random access memory cell comprising: a magnetoelectricjunction bit comprising: a ferromagnetic free layer; a ferromagneticfixed layer; and a dielectric layer interposed between the ferromagneticfree layer and the ferromagnetic fixed layer; wherein themagnetoelectric junction bit is configured such that when a voltage of agiven polarity is applied across the magnetoelectric junction bit forhalf a precessional period of the ferromagnetic free layer, theperpendicular magnetic anisotropy and magnetic coercivity of theferromagnetic free layer are reduced through a voltage controlledmagnetic anisotropy effect and the magnetization of the ferromagneticfree layer changes direction; and wherein the magnetoelectric junctionbit is configured such that when the applied voltage of the givenpolarity is reduced before the end of the half precessional period ofthe ferromagnetic free layer, a magnetic torque of the ferromagneticfree layer is reduced before a maximum magnetic trajectory of theferromagnetic free layer is reached.
 12. The magnetoelectric randomaccess memory cell of claim 11, further comprising: a first terminalcoupled to a bit line; a second terminal coupled to a source line; and athird terminal coupled to a word line.
 13. The magnetoelectric randomaccess memory cell of claim 12, wherein the magnetoelectric junction bitis coupled to the drain of an MOS transistor.
 14. The magnetoelectricrandom access memory cell of claim 13, wherein the voltage of the givenpolarity is applied using a pulse generator.
 15. The magnetoelectricrandom access memory cell of claim 14, wherein the pulse generator isselected from the group consisting of a bit line driver, a source linedriver, and a word line driver.
 16. The magnetoelectric random accessmemory cell of claim 15, wherein: the rising edge of the application ofthe voltage of the given polarity decreases the perpendicular magneticanisotropy and causes a precessional motion of magnetization between twostates of the ferromagnetic free layer, wherein the magnetizationdirection of the ferromagnetic free layer is different between the twostates; and the falling edge of the application of the voltage of thegiven polarity restores the decrease in the perpendicular magneticanisotropy and stops the precessional motion of magnetization.
 17. Themagnetoelectric random access memory cell of claim 16, wherein a voltageof the polarity opposite the given polarity is applied across themagnetoelectric junction bit subsequent or near simultaneously with thefalling edge of the application of the voltage of the given polarity toincrease the perpendicular magnetic anisotropy of the ferromagnetic freelayer.
 18. The magnetoelectric random access memory cell of claim 17,wherein the voltage of the polarity opposite the given polarity isapplied using capacitive coupling from the word line to themagnetoelectric junction bit through the gate-to-source-capacitance. 19.The magnetoelectric random access memory cell of claim 17, wherein thevoltage of the polarity opposite the given polarity is applied throughgenerating a negative voltage with respect to ground on the bit linewhile keeping a voltage of the source line at ground level.
 20. Themagnetoelectric random access memory cell of claim 17, wherein thevoltage of the polarity opposite the given polarity is applied bygenerating a positive voltage pulse on the source line after a writevoltage pulse on the bit line.